The invention relates to a control unit for a brushless motor, comprising
position detection means for generating a signal indicative of the position of the motor,
supply means for energizing two or more coils of the motor in response to said signal,
output terminals for electrically coupling the coils to the supply means.
The invention further relates to a device for reading and/or writing an information carrier, which device includes such a control unit.
Such a control unit is known from U.S. Pat. No. 5,783,920. The control unit controls a motor having three windings, one of the three windings being alternately non-energized. The back EMF of the non-energized winding is measured. This signal is used for synchronizing a voltage-controlled oscillator (VCO). The VCO controls a state machine, which controls the winding energization. The known control unit has the advantage that it does not require a separate sensor for measuring the position of the motor. However, a disadvantage of this control unit is that the alternate energization and non-energization of the windings produces annoying noises. It is therefore desirable to drive the windings with a pulse-width modulated signal whose pulse width varies in a sinusoidal fashion, the signal being modulated with a non-audible frequency, for example higher than 20 kHz, for example 30 kHz. However, the time for measuring the back EMF is then inadequate.
It is an object of the invention to provide a control unit which enables the motor windings to be driven with a pulse-width modulated signal without a separate sensor being necessary.
For this purpose the control unit according to the invention is characterized in that the position detection means include polarity detection means for generating a signal which is a measure of the direction of the current through at least one of the coils. When the speed of the motor is sufficiently high there is a reliable relationship between the polarity of the current through the winding and the position of the motor. The polarity of the current can be measured easily regardless of whether the winding is energized or is not energized.
An attractive embodiment is characterized in that the supply means comprise a first series arrangement of controllable semiconductor elements, an output terminal being coupled to a first node of the semiconductor elements, and the polarity detection means comprise a second series arrangement of controllable semiconductor elements, which second series arrangement has a second node, and the semiconductor elements in the second series arrangement being driven in synchronism with the semiconductor elements in the first series arrangement, which polarity detection means further include voltage comparison means having a first input coupled to the first node and having a second input coupled to the second node. In this embodiment the polarity detection means and the supply means can easily be integrated together in one IC.
Preferably, the control unit in accordance with the invention is characterized by modulating means for generating a pulse-width modulated control signal for controlling the supply means. By energizing the motor with a pulse-width modulated supply voltage, using a modulation frequency which is substantially higher than the commutation frequency of the motor, a silent drive is obtained. However, the invention likewise applies to control means which do not include such modulation means.
In an attractive embodiment of the control unit in accordance with the invention the supply means comprise a phase-locked loop.
A favorable variant of this embodiment of the control unit in accordance with the invention is characterized in that the phase-locked loop includes logic combination means, such as an EXOR, a NAND or a NOR gate, for determining a phase difference between the polarity signal and a feedback signal of the phase-locked loop. In this variant the setting of the phase-locked loop is comparatively immune to an offset in the polarity detector and to interference as a result of the high-frequency modulation of the supply voltage of the motor. Alternatively, the phase detector may be of the sequential type, such as a D flip-flop, an SR flip-flop.
The control unit is particularly suitable for use in device in accordance with the invention for reading and/or writing a rotating information carrier. A device in accordance with the invention includes a transducer for converting a pattern recorded on the information carrier into a read signal, or for recording a pattern on the information carrier in response to a write signal, which device includes a brushless motor for rotating the information carrier and a control unit in accordance with the invention, for controlling the motor.